1. Field of the Invention
The invention is related to the field of well logging instrumentation. More specifically, the invention is related to mechanisms for applying lateral force to the housing of a well logging instrument such as a caliper arm or similar device, and mechanisms for coupling the motion of the caliper arm to a sensor for determining the amount of lateral extension of the caliper arm from the instrument housing.
2. Description of the Related Art
Many types of well logging instruments include caliper or back-up arms which selectably extend radially outward from the instrument housing and are placed in contact with the wall of a wellbore into which the instrument is inserted.
In many well logging instruments which have caliper or back-up arms, the arm is typically coupled to some type of resilient mechanism for pressing the arm into contact with the wall of the wellbore. In some logging instruments this mechanism can be strong enough to cause the instrument housing to be pushed into contact with the wall of the wellbore in reaction to the force exerted on the arm. Other instruments remain in the center of the wellbore between circumferentially spaced apart arms each forced into contact with the wellbore through a resilient mechanism. Other arm extending mechanisms are only strong enough to cause the arm itself to remain in contact with the wellbore wall. Examples of such well logging instruments including both types of arm-extending mechanisms can be found, for example in U.S. Pat. No. 4,614,250 issued to Panetta et al, in U.S. Pat. No. 4,715,440 issued to Boxell et al and in U.S. Pat. No. 5,528,556 issued to Delpuech et al. Most of the mechanisms known in the art are selectably controllable by the system operator, and can be fully retracted at the command of the system operator to facilitate insertion and removal of the well logging instrument from the wellbore.
In most types of arm-extending mechanisms, the arm is generally coupled to a sensor which can determine the amount of radial separation between the instrument housing and the end of the arm. The measurements made by this sensor can be scaled into measurements of the internal diameter of the wellbore at the point of contact between the arm and the wall of the wellbore. Prior art mechanisms for causing the arm to exert force, and prior art mechanisms for coupling the motion of the arm to the sensor typically required the use of various types of seals to exclude fluids in the wellbore from entering the main housing of the well logging instrument. The fluids in the wellbore are typically under very high hydrostatic pressure. The seals are therefore subject to frequent failure, allowing fluids in the wellbore to enter the instrument and damage it. These seals are also difficult and expensive to maintain because of the very high pressures that they must withstand.
Magnetic couplings are known in the art for transferring motion of a "driving" element to a "driven" element through a non-magnetic housing or container without the need to create a sealed passage through the wall of the housing. See for example, U.S. Pat. No. 3,245,665 issued to Steel and U.S. Pat. No. 4,120,618 issued to Klaus. Still another such magnetic coupling is shown in U.S. Pat. No. 5,209,699 issued to Hashimoto et al. The device shown in Hashimoto et al '699 is also intended to couple rotary motion without creating a passage through the wall of a vacuum chamber.
All of these prior art magnetic couplings, however, are adapted to transfer rotary motion between the driving element and the driven element, and at least in the case of the device shown in Hashimoto et al '699, to do so with a very high degree of synchronization between the movement of the driving element and the movement of the driven element. In the case of the well logging instruments including arm-extending mechanisms, however, the overall motion of linkages connecting the back-up arm to the sensor and to the driving device (the arm-forcing mechanism) is typically linear. The magnetic couplings known in the art do not provide appropriate means for coupling linear motion through the wall of a housing so as to be useful for transferring motion for caliper or back-up arms on well logging instruments. Positioning of permanent magnets or magnetic couplings on either side of the housing wall of a well logging instrument would provide some coupling of motion between the internal and external elements, but without more such a coupling would not provide enough synchronization of motion between the internal and external elements to be useful with a motion sensor to determine the internal diameter of the wellbore.